i dont know bout a blow by check, but the best thing i suggest is a CYLINDER LEAKAGE Test. this should tell you exactly where your losing compression without having to whip the head off!
its a very simple but effective test!
like a compression test, you have to fit an adaptor to the spark plug hole,
but instead of fitting a guage to it, you plug an airline to it! you must ENSURE that the cylinder that you are testing, the piston MUST BE AT T.D.C
otherwise the pressue from the airline will simply send the piston down!
at T.D.C the valves are shut and the con rod is dead straight / vertical, so the pressure will just sit in the combustion chamber! NOW all you do is listen for where you are losing pressure!
if you can FEEL / HEAR AIR coming out of the following:
Oil filler cap = Rings
Exhaust Pipe = Exhaust Valves
Inlet Manifold / Throttle = Inlet Valves
Header Tank = Headgasket